In the drilling of earth boreholes, the term "straight hole drilling" commonly means no active effort is being made to change the course of a progressing bore, whatever the existing angle or direction of the hole may be. Directional drilling efforts are generally construed to mean efforts are being made by active devices downhole to change the existing course of a progressing well bore.
There are many devices commonly used downhole to change the course of a bore, usually defined as the angle off vertical and the direction relative to an azimuthal earth reference, usually north. Currently, the most effective means to change course of a bore is to use a bent drill string component just above a fluid powered downhole drilling motor. The bent component is referred to as a bent sub. In softer formations being drilled, a jet on one side of a drill bit can be used to cause a progressing hole to change course. The jet has to be oriented relative to an azimuthal earth reference, if the direction favored is to be controlled. A bent sub, if used with the jet system, allows the drill string to follow through the curve in the borehole the jet tends to produce.
Drilling with a bent sub commonly requires only a few hours to achieve the course control purpose. The drill string is then commonly removed from the hole to remove the bent sub, and the drill string is again assembled with or without the downhole motor or jet, so that straight hole drilling may continue. This is called "tripping the string" and may take more than ten hours.
It is highly desirable to avoid tripping the drill string, and to that end, several efforts have been made over several years. The best known method accomplishing the effect of changing from a bent sub to a straight sub without tripping the drill string involved a form of gimbal, or hinge, in the drill string that was controllable. Control of the gimbal was exercised by dropping a spear down the drill string bore to hold the gimbal straight and recovering the spear by a wire line to permit the controlled device to effectively bend. The bend, or deflection, of the drill string rarely exceeded three degrees. The exercise of the spear controlled device consumed considerable time because of the trips down the pipe bore with the wire line.
The need to conveniently change, by simple actions of the earth surface, the configuration of downhole equipment to permit alternate straight hole and directional drilling, has persisted for years. More frequent conversion to directional drilling configuration reduces the required angle of pipe string centerline deflection to something like one degree. This permits construction of more rugged gimbals, which are prone to weakness. Additionally, frequent smaller corrections of well bore direction produces a better hole, more easily cased and serviced.
Apparatus of this invention, utilizing the Control Selector Valve of my copending application Ser. No. 784,262, has recently been used in downhole well drilling operations.